1. Field of the Invention
This invention relates in general to borehole measurement and telemetry systems whereby ambient parameters of a well bore are measured downhole and telemetered uphole. In particular, the invention relates to a system in which but two wires are used to power and communicate with the downhole instrumentation and telemetry circuitry.
2. Description of the Prior Art
Downhole measurement and instrumentation systems are known especially in the field of well logging. However, the invention described herein relates to providing a downhole measurement system which may be placed in a producing well to provide a continuous recording of a downhole parameter for the life of the producing well.
Providing electrical leads to a downhole instrumentation system in a producing well can be a source of difficulty for oil well owners. It is possible to run hydraulic fluids and electrical conductors by means of elastomeric encapsulated conduits running between the wellhead and the downhole instrument package but the more conductors that are provided, the more expensive is such a conduit package. Therefore, it has been desirable in the art to provide an instrumentation and telemetry system having but two conductors reaching between wellhead electronics and a downhole instrumentation package. One of the problems that has faced inventors in this field has been providing downhole DC power over such a long distance as may be required to reach from the wellhead to the instrumentation package. For example, many wells reach from 5,000, 10,000, 15,000 and more feet. Therefore, any electrical leads that reach to such depths inherently have high ohmic losses due to their length. In order to avoid high IR losses due to high current type supply systems, it has been known in the prior art to provide high voltage via a pair of leads for supplying power in one direction and multiplexing signals back in the other.
However, such a system inherently requires the use of transformers and due to the limited space of such a downhole probe or instrumentation package, very small transformers have had to be provided. In order to provide sufficient inductive coupling through such transformers, they have required the use of an iron core. Small iron core transformers can tolerate only a very small imbalance in DC currents flowing in the primary and secondary windings due to magnetic saturation of the small iron core if imbalance occurs. Such saturation inherently disrupts the linear coupling of any AC signal that is required to be telemetered through such a transformer back up the electrical leads to wellhead instrumentation.
Another problem faced by the prior art has been the generation of pure sine waves downhole for the representation of at least two downhole parameters such as pressure and temperature. Pure sine waves representing both parameters are desired to be multiplexed together and then applied through the downhole transformer and up the pair of electrical conductors back to the surface.
3. Identification of the Objects of the Invention
It is therefore a primary object of the invention to provide a two wire instrumentation and telemetry system for measuring at least two parameters downhole and sending a multiplexed signal representative of those two parameters via said two wires back to the surface while overcoming the problems faced by prior art designers.
Another object of the invention is to provide a current balancing circuit for a downhole instrumentation system in which the current flowing through one side of a small iron core transformer is sensed and a substantially equal and opposite current is generated in response thereto for applying to the other winding of the transformer in order to avoid magnetic saturation of the core of the transformer.
Still another object of the invention is to provide a novel circuit to generate a sine wave having its fundamental frequency identical to a square wave the frequency of which represents a parameter to be measured.
Still another object of the invention is to provide a system for measuring the temperature and pressure ambient at the bottom of the well and telemetering a signal representative of temperature and pressure to the top of the well.
It is another object of the invention to provide electronic circuits responsive to the signals transmitted to the top of the well and generating standard four to twenty milliampere signals representing the two measured quantities at the bottom of the well.